1. Field of the Invention
This application concerns the field of clutch drive mechanisms, particularly suitable for vertical venetian blinds, having a plurality of slats in which each slat is provided with a carriage mounted in a top rail. In such blinds, a drive shaft is provided in the top rail and a clutch mechanism is interposed between the drive shaft and each slat, in order to rotate each slat about its vertical axis relative to its carriage. Various gear and clutch mechanisms have been proposed for transmitting the motion of the drive shaft to the slats and the present invention is directed to such a clutch mechanism.
2. Prior Art
One known means for achieving the rotation of the slats to open and close the venetian blind disclosed in U.S. Pat. No. 3,996,988 comprises a drive shaft extending through each of the carriages which drives a sliding clutch that, in turn, rotates the slat. The clutch includes a first bushing slidably mounted on the drive shaft but non-rotatable with respect to the shaft. The bushing has an outer surface that is other than round provided by a plurality of reinforcing enlargements on its outer surface. These enlargements are in friction contact with the inner surface of a bore in a worm gear. The friction between these enlargements and the bore in the worm gear is sufficient to provide rotation of the worm and thus rotation of the slats; however, when the worm reaches a stop provided for the purpose, further rotation of the worm gear is prevented. At this point the friction between the enlargements on the inner bushing and the inner surface of the bore is overcome by the torque applied to the inner bushing by the drive shaft. Accordingly, the inner bushing continues to rotate by slipping within the bore of the now stationary worm gear.
In actual practice, it has been found that this design has a number of drawbacks. In order for the clutch of this design to work well, the dimensions of the inner surface of the bore in the worm gear and the enlargements on the bushing must be maintained within very small tolerances. Indeed, in series production of venetian blinds, it has been found exceedingly difficult to maintain the tolerances required for proper operation of the blind.